Transcript CHM 434F/1206F SOLID STATE MATERIALS CHEMISTRY

DIRECT VISUALIZATION OF ATOM SCALE DEFECTS BY
TRANSMISSION ELECTRON MICROSCOPY
VISUALIZING MISFIT DISLOCATIONS AT INTERFACE
BETWEEN MBE DEPOSITED Si AND GaAs SUBSTRATE
MISFIT DISLOCATION
111 FAULTS
SURFACE STRUCTURE AND REACTIVITY EFFECTS
ON DIRECT REACTION OF SOLIDS
Mg 2+
O2Mg 2+
O2-
Mg 2+
Mg 2+
O2-
O2-
Mg 2+
O2-
Mg 2+
O2-
Mg 2+
Mg 2+
Mg 2+
Mg 2+
Mg 2+
Mg 2+
{100} face
{111} face
Mg 2+
O2-
Mg 2+
Mg 2+
Mg 2+
O2-
Mg 2+
Mg 2+
O2O2-
O2O2-
O2O2-
O2O2-
O2O2-
{111} face
Nucleation depends on surface structure of reacting phases - crystal faces in
contact - MgO rock salt - different Miller index faces exposed - ion arrangements
in crystal face different - also distinct crystal habits (octahedral, cubooctahedral,
cubic) possible depending on growth conditions and additives - {100} alternating
Mg(2+), O(2-) at corners of square grid - {111} Mg(2+) or O(2-) in hexagonal
arrangement - implies different surface structures and reactivities
FACTORS WHICH CONTROL CRYSTAL
GROWTH AND MORPHOLOGY
Needle growth
Platelet growth
{111} vs {100} growth
rates: cube, cubooctahedral
or octahedral shape
Most prominent surfaces exhibit slower growth
Growth rate of specific surfaces controls morphology of crystal
Depends on area of a face - structure of exposed face accessibility of a face - surface energy - surface reconstruction
- adsorption at surface sites - surface defects
FACTORS WHICH CONTROL CRYSTAL
GROWTH AND MORPHOLOGY
Needle growth
Platelet growth
{111} vs {100} growth
rates: cube, cubooctahedral
or octahedral shape
All types of defects, intrisic or extrinsic, vacancies, interstitials,
lines, planes, dislocations, grain boundaries, enhance diffusion
of ions and crystal growth rates
Defects play major role in reactivity, nucleation, crystal growth,
materials properties (electronic, optical, magnetic, chargetransport, mechanical, thermal)
CRYSTAL NUCLEATION AND
GROWTH
DG°(crystal) = DG° (surface) - DG° (bulk)
Induction period - growth of viable crystal nuclei
Growth and dissolution of seed
Equilibrium growth condition when DG°(crystal) = 0
DG° (surface) = DG° (bulk)
Condition to creates critical size nuclei
Crystal growth favored when DG° (surface) < DG°
(bulk)
• Sigmoid-shaped nucleation-growth-depletion curve
• Large crystals grow at expense of small ones
• Crystal growth ceases when nutrients depleted
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ABALONE SHELL - BIOMINERALIZATION - VECTORIAL
CONTROL OF CRYSTAL NUCLEATION, GROWTH AND
FORM IN NATURE
95wt% inorganic - site specific calcite platelet oriented growth
5wt% b-sheet protein organic structure directing matrix
Organic-inorganic nanocomposite 1000x fracture toughness of
bulk calcite – impact energy on shell dissipated in soft protein
layers rather than in hard calcite preventing cracking - learning
from Nature - biomimetic inorganic materials chemistry technology transfer from biology
“SHAKE-AND-BAKE” SOLID STATE SYNTHESIS
• Although this approach may seem to be ad hoc
and a little irrational at times, the technique has
served solid state chemistry well over the past 50
years
• It has given birth to the majority of high
technology devices and products that we take for
granted every day of our lives
• Thus it behooves us to look critically and
carefully at the methods used if one is to move
beyond to the new chemistry and a rational
synthesis of materials
THINKING ABOUT REAGENTS
• Drying reagents MgO/Al2O3 200-800°C, maximum
SA
• In situ decomposition of precursors at 600-800°C
MgCO3/Al(OH)3  MgO/Al2O3
• Intimate mixing of precursor reagents
• Homogenization of reactants using organic
solvents, grinding, ball milling, ultrasonification
CONTAINER MATERIALS
• Chemically inert crucibles, boats
• Noble metals Nb, Ta, Au, Pt, Ni, Rh, Ir
• Refractories, alumina, zirconia, silica, boron nitride,
graphite
• Reactivity with containers at high temperatures
needs to be carefully evaluated for each system
SOLID STATE SYNTHESIS HEATING PROGRAM
• Furnaces, RF, microwave, lasers, ion and electron
beams
• Prior decompositions and frequent cooling,
grinding, boost SA of reacting grains
• Overcoming sintering, grain growth, brings up SA,
fresh surfaces, enhanced contact area
• Pellet and hot press reagents - higher surface
contact area, enhances rate, extent of reaction
• Care with unwanted preferential component
volatilization if T too high, composition dependent
• Need controlled atmosphere for unstable oxidation
PRECURSOR SOLID STATE SYNTHESIS METHOD
• Co-precipitation, high degree of homogenization,
high reaction rate - applicable to nitrates, acetates,
oxalates, alkoxides, b-diketonates
• Concept: precursors to magnetic spinels - recording
media
• Zn(CO2)2/Fe2[(CO2)2]3/H2O 1 : 1 mixing
• H2O evaporation, salts co-precipitated - solidsolution mixing on atomic scale, filter, calcine in air
• Zn(CO2)2 + Fe2[(CO2)2]3  ZnFe2O4 + 4CO + 4CO2
• High degree of homogenization, lower reaction
temperature, faster rate
PROBLEMS WITH CO-PRECIPITATION METHOD
• Co-precipitation applicable to nitrates, acetates,
oxalates, alkoxides, b-diketonates and so forth
requires:
• Similar salt solubilities
• Similar precipitation rates
• Avoid super-saturation as poor control of coprecipitation
• Useful for spinels
• Disadvantage: difficult to prepare high purity,
accurate stoichiometric phases
DOUBLE SALT PRECURSORS
• Known stoichiometry double salts having
controlled stoichiometry
• Ni3Fe6(CH3CO2)17O3(OH).12Py
• Basic double acetate pyridinate
• Burn off organics at 200-300oC, then calcine at
1000oC in air for 2-3 days
• Product highly crystalline phase pure NiFe2O4
spinel
DOUBLE SALT PRECURSORS
• Chromite spinel Precursor compound
oC
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MgCr2O4
NiCr2O4
MnCr2O4
CoCr2O4
CuCr2O4
ZnCr2O4
FeCr2O4
(NH4)2Mg(CrO4)2.6H2O
(NH4)2Ni(CrO4)2.6H2O
MnCr2O7.4C5H5N
CoCr2O7.4C5H5N
(NH4)2Cu(CrO4)2.2NH3
(NH4)2Zn(CrO4)2. 2NH3
(NH4)2Fe(CrO4)2
Ignition T,
1100-1200
1100
1100
1200
700-800
1400
1150
Good way to make chromite spinels, important tunable magnetic materials juggling electronic-magnetic properties of the A Oh and B Td ions in the
spinel lattice
PEROVSKITE FERROELECTRICS
BARIUM TITANATE
• Control of grain size determines ferroelectric
properties, important for capacitors,
microelectronics
• Direct heating of solid state precursors is of limited
value in this respect
• BaCO3(s) + TiO2(s)  BaTiO3(s)
• Sol-gel reagents useful to create single source
barium titanate precursor with correct stoichiometry
BASICS: FERROELECTRIC BARIUM TITANATE
Cubic perovskite BaTiO3
Tetragonal perovskite BaTiO3
Small grains, tetragonal
to cubic surface
gradients,
ferroelectricity particle
size dependent
Multidomain paraelectric above Tc
Cooperative electric dipole
interactions within each domain
Multidomain ferroelectric below Tc
Single domain superparaelectric
SINGLE SOURCE PRECURSOR SYNTHESIS OF
BARIUM TITANATE - FERROELECTRIC MATERIAL
• Ti(OBu)4(aq) + 4H2O  Ti(OH)4(s) + 4BuOH(aq)
• Ti(OH)4(s) + (COO)22-(aq)  TiO(COO)(aq) + 2OH-(aq) + H2O
• Ba2+(aq) + (COO)22-(aq) + TiO(COO)(aq)  Ba[TiO(COO)2](s)
• Precipitate contains barium and titanium in correct ratio
and at 920C decomposes to barium titanate according to:
• Ba[TiO(COO)2](s) BaTiO3(s) + 2CO(g)
• Grain size important for control of ferroelectric properties
• Used to grow single crystals hydrothermally
SOL-GEL SINGLE SOURCE PRECURSORS TO LITHIUM
NIOBATE - NLO MATERIAL
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LiOEt + EtOH + Nb(OEt)5  LiNb(OEt)6  LiNbO3
LiNb(OEt)6 + H2O  LiNb(OEt)n(OH)6-n   gel
LiNb(OEt)n(OH)6-n + D+O2  LiNbO3
Lithium niobate, ferroelectric perovskite,
nonlinear optical NLO material, used as
electrooptical switch
Bimetallic alkoxides - single source precursor
Sol-gel chemistry - hydrolytic polycondensation
MOH + M’OH  MOM’ + H2O
Yields glassy product
Sintering product in air - induces crystallization
INDIUM TIN OXIDE -ITO
• Indium sesquioxide In2O3 (wide band gap
semiconductor) electrical conductivity enhanced by
p-doping with (10%) Sn(4+), ITO is SnnIn2-nO3
• ITO is optically transparent, electrically conducting,
thin films are vital as electrode material for solar
cells, electrochromic windows/mirrors, LEDs,
electronic ink
• Precursors - EtOH solution of (2n)In(OBu)3/nSn(OBu) 4
• Hydrolytic poly-condensation to form gel, spin coat
gel onto glass substrate to make thin film
• Dry gel at 50-100C, heat at 350C in air to produce
SUB -10 NM NANOSCALE
DIRECT SOLID STATE REACTION
Electron Beam Nanolithography Using Spin-Coatable TiO2 Resists
• Utilization of spin-coatable
sol gel based TiO2 resists by
chemically reacting titanium
n-butoxide with
benzoylacetone in methyl
alcohol.
Choosing the right
solid state precursor
• They have an electron beam
sensitivity of 35 mC cm-2 and
are >107 times more
sensitive to an electron
beam than sputtered TiOx
and crystalline TiO2 films.
Sub-10 nm Electron Beam Nanolithography
Using Spin-Coatable TiO2 Resists
• Fourier transform infrared
studies suggest that exposure
to an electron beam results in
the gradual removal of organic
material from the resist.
Choosing the right
solid state precursor
• This makes the exposed resist
insoluble in organic solvents
such as acetone, thereby
providing high-resolution
negative patterns as small as 8
nm wide. Such negative
patterns can be written with a
pitch as close as 30 nm.
Nanometer scale precision structures
Nanoscale TiO2 structures offer new opportunities for developing next
generation solar cells, optical waveguides, gas sensors, electrochromic
displays, photocatalysts, photocatalytic mCP, battery materials
Nanometer scale tolerances
MAGNETIC GARNETS, YxGd3-xFe5O12
TUNABLE MAGNETIC MATERIALS
• Y(NO3)3 + Gd(NO3)3 + FeCl3 + NaOH  YxGd3-xFe5O12
• Mixed metal hydroxide aqueous precursor synthesis
method, reactants red brown, solid products olive
green
• Firing pellets at 900oC, 18-24 hrs, regrinding,
repelletizing, repeated firings, removes REFeO3
perovskite impurity
• PXRD used to identify garnet phase, detects any
crystalline impurity phase like REFeO3, enables UC
PXRD OF SOLID PRODUCTS OF Y(NO3)3 + Gd(NO3)3 +
FeCl3 + NaOH REACTION
HYDROTHERMAL SYNTHESIS AND CRYSTAL
GROWTH OF YTTRIUM ALUMINUM GARNET
aqueous basic medium, mineralizes,
temperature gradient, transports,
deposits reactants on seed crystal to grow
product yttrium aluminum oxide crystal
baffles
T2
T1
Al2O3
T2
Y2O3
Seed crystal to grow Y3Al5O12 crystal
GARNETS DISPLAY INTERESTING COOPERATIVE
MAGNETIC BEHAVIOR
• Tunable magnet by varying magnetic superlattice
components without disrupting garnet structure
• Similar idea to magnetic spinel AB2O4 solid solution
behavior - in which one has magnetically tunable Td
(A) and Oh (B) metal sites
• Rare earth garnets R3Fe5O12
• General Formula C3A2D3O12 (8 formula units per
cubic unit cell - total 160 atoms)
ONE OCTANT OF CUBIC UNIT CELL OF YAG
Faces 3 dodecahedral Y(3+) sites
Corners and center 2Oh AlO6 sites
Faces 3Td AlO4 sites
One octant of cubic unit cell of garnet
GARNETS DISPLAY INTERESTING COOPERATIVE
MAGNETIC BEHAVIOR
• C3A2D3O12 isomorphous replacement of Y(3+) for
Gd(3+) on dodecahedral C cation sites (works for all
rare earths except La, Ce, Pr, Nd)
• Forms solid solution as similar ionic radii,
• R(Gd(3+) = 0.938Å > R(Y(3+) = 0.900Å
• Complete family accessible, YxGd3-xFe5O12, 0  x  3
• 2Fe(3+) Oh A-sites, 3Fe(3+) D-Td sites, 3RE(3+) C
dodecahedral sites
MODELS FOR DETERMINING THE Y(3+)/Gd(3+)
DISTRIBUTION IN YxGd3-xFe5O12
1. Solid solution - random distribution of two
components - EDX mapping
2. Physical mixture of two end members - phase
segregation - PXRD
3. Compositional gradients - STEM imaging - EDX
mapping
4. Core-corona - cherry model - surface free energy
driven - EDX mapping
5. Domains smaller than 10 nm - PXRD line broadening
6. Ordered superlattice - ED
MODELS FOR DETERMINING THE Y(3+)/Gd(3+)
DISTRIBUTION IN YxGd3-xFe5O12
• Interesting problem in solid state materials
characterization
• If any measured physical property P of the product
follows Vegard law behavior this defines a solid
solution for the Y(3+)/Gd(3+) distribution
• P(YxGd3-xFe5O12) = Px/3(Y3Fe5O12) + P(3x)/3(Gd3Fe5O12)
• Measured P of product is the atomic/mole fraction
weighted average P of the end-member materials
MAGNETIC GARNETS, YxGd3-xFe5O12
TUNABLE MAGNETIC MATERIALS
• Cubic unit cell parameter a versus x for YxGd3-xFe5O12
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Composition
Y3Fe5O12
Y2.5Gd0.5Fe5O12
Y2Gd1Fe5O12
Y1.5Gd1.5Fe5O12
Y1Gd2Fe5O12
Y0.5Gd2.5Fe5O12
Gd3Fe5O12
Lattice parameter, nm
1.2370
1.2382
1.2402
1.2423
1.2437
1.2450
1.2468
R(Gd(3+)) = 0.938Å > R(Y(3+)) = 0.900Å
MAGNETIC GARNETS, YxGd3-xFe5O12
TUNABLE MAGNETIC MATERIALS
• Isomorphous random replacement of Y3+ for Gd3+on
dodecahedral sites of cubic lattice
• Vegard law behavior
• P(YxGd3-xFe5O12) = Px/3(Y3Fe5O12) + P(3x)/3(Gd3Fe5O12)
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Any property of a solid-solution member is the atom/mole fraction
weighted average of the end-members - distinguishes statistical from
other types of mixtures (core-corona, phase separation, domains,
gradients, superlattices)
• Cubic lattice parameter a shows linear Vegard law
behavior with x
TUNABLE MAGNETIC PROPERTIES BY VARYING x IN
THE BINARY GARNET YxGd3-xFe5O12
• Counting electrons and unpaired electron spins
• x dodec Y(3+) sites
4d0, 4f0
• (3-x) dodec Gd(3+) sites
• 3 Td Fe(3+) sites
• 2 Oh Fe(3+) sites
5UPEs
0 UPEs
HS 4f7
HS 3d5
HS 3d5
7 UPEs
5 UPEs
TUNABLE MAGNETIC PROPERTIES BY VARYING x
IN THE BINARY GARNET YxGd3-xFe5O12
• Ferrimagnetically coupled material, oppositely
aligned electron spins on Td and Oh Fe(3+) magnetic
sub-lattices
• Counting spins Y3Fe5O12
• 3 x 5 - 2 x 5 = 5UPEs
ferrimagnetic at low T
• Counting spins Gd3Fe5O12 ferrimagnetic at low T
• 3 x 7 -3 x 5 + 2 x 5 = 16UPEs
• Tunable magnetic garnet from 16 to 5 UPEs
VEGARD LAW AT THE NANOSCALE
SYNTHESIS OF COMPOSITION TUNABLE MONODISPERSE CAPPED
ZnxCd1-xSe ALLOY NANOCRYSTALS
Spatial and quantum confinement and
dimensionality